docs: def is_valid(self):
          """
          Check integrity and validity of this frame.
  
          :return: bool True if this frame is structurally valid.
          """
          conditions = [
              self.protocol_id == 0,  # Modbus always uses protocol 0
              2 <= self.length <= 260,  # Absolute length limits
              len(self.data) == self.length - 2,  # Total length matches data length
          ]
          return all(conditions)